It is called solving the equation.
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I would suggest that the answer is "evaluating it".
Evaluating the expression
When a given set of values for the variables are substituted in the expression the result is the value of the expression.
It means finding the value of the expression.
You try finding a factor. If there is no proper factor, then it is a prime.
It means finding a simpler expression. For example, 3x + 4x can also be written as 7x (I simply added the 3 and the 4, which is valid because of the common factor x). Or, if you have a fraction like 12/36, you can simplify it as 1/3. Or a fraction with variables, like (3x2 + 4x) / x can be simplified as 3x + 4. Review your algebra book for additional examples.
Evaluating the expression
Evaluating it.
When a given set of values for the variables are substituted in the expression the result is the value of the expression.
Dependent. I think.
The slope of a line is the same thing as the rate of change between two variables in a linear relationship.
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Solving the equation.
It means finding the value of the expression.
The variables stand for an unknown number that has not yet been identified which has been kept as a variable for the purpose of finding the value of.
It is about finding a value of the variable (or variables) that make the equation a true statement.
Finding a set of value for the set of variables so that, when these values are substituted for the corresponding variables, all the equations in the system are true statements.